Automatic measurement of dimensional data with a laser tracker

Abstract

Measuring with a system having retroreflector targets and a laser tracker includes storing a list of nominal coordinates for three targets and at least one added point; capturing on a photosensitive array of the tracker a portion of the light emitted by a light beam and reflected off the three targets; obtaining spot positions on a photosensitive array of a tracker camera from light reflected off the three targets; determining a correspondence between three spot positions on the tracker photosensitive array and the nominal coordinates of the three targets; directing a beam of light from the tracker to the three targets based at least in part on the nominal coordinates of the first target and the first spot position; measuring 3-D coordinates of the three targets with the tracker; determining 3-D coordinates of the at least one added point based at least in part on the measured 3-D coordinates of the three targets and the nominal coordinates of the at least one added point.

What is claimed is: 
     
       1. A method for measuring with a system, the method comprising steps of:
 providing the system including a collection of retroreflector targets and a laser tracker, the collection of retroreflector targets including at least three non-collinear retroreflector targets, the at least three non-collinear retroreflector targets including a first target, a second target, and a third target, the laser tracker in a first frame of reference fixed with respect to tracker surroundings, the laser tracker having a structure, a first light source, an absolute distance meter, a first angular transducer, a second angular transducer, a tracking system, a first camera, a second light source, and a processor, the structure rotatable about a first axis and a second axis, the first light source producing a first light beam that cooperates with the absolute distance meter, the first angular transducer measuring a first angle of rotation about the first axis, the second angular transducer measuring a second angle of rotation about the second axis, the tracking system configured to move the first light beam to a center of any retroreflector target from among the collection of retroreflector targets, the first camera including a first lens system and a first photosensitive array, the second light source providing a second light beam, and the processor configured to operate the laser tracker; 
 storing a list of nominal coordinates for the first target, the second target, the third target, and at least one additional point, the nominal coordinates being three-dimensional coordinates in a second frame of reference; 
 capturing on the first photosensitive array a portion of the light emitted by the second light beam and reflected off the first target, the second target, and the third target; 
 obtaining spot positions on the photosensitive array from the portion of light reflected off each of the first target, second target, and the third target; 
 determining a correspondence between a first spot position, a second spot position, and a third spot position on the first photosensitive array and the nominal coordinates of the first target, the second target, and the third target, respectively; 
 directing the first beam of light to the first target based at least in part on the nominal coordinates of the first target and the first spot position; 
 measuring three-dimensional coordinates of the first target using the absolute distance meter, the first angular transducer, and the second angular transducer; 
 directing the first beam of light to the second target based at least in part on the nominal coordinates of the second target and the second spot position; 
 measuring three-dimensional coordinates of the second target using the absolute distance meter, the first angular transducer, and the second angular transducer; 
 directing the first beam of light to the third target based at least in part on the nominal coordinates of the third target and the third spot position; 
 measuring three-dimensional coordinates of the third target using the absolute distance meter, the first angular transducer, and the second angular transducer; 
 determining three-dimensional coordinates of the at least one additional point in the first frame of reference based at least in part on the measured three-dimensional coordinates of the first target, the second target, the third target, and the nominal coordinates of the at least one additional point; 
 storing the determined three-dimensional coordinates of the at least one additional point; 
 providing a maximum allowable discrepancy; 
 providing a coefficient of thermal expansion for an object under test; 
 providing a reference temperature; 
 placing a first reference retroreflector and a second reference retroreflector on the object under test, there being a first distance between the first reference retroreflector and the second reference retroreflector at the reference temperature; 
 measuring a temperature of the object under test; 
 calculating a first temperature difference by subtracting the reference temperature from the measured temperature of the object under test; 
 calculating a scale factor by multiplying the first temperature difference by the coefficient of thermal expansion; 
 measuring the three-dimensional coordinates of the first reference retroreflector using the absolute distance meter, the first angular transducer, and the second angular transducer; 
 measuring the three-dimensional coordinates of the second reference retroreflector using the absolute distance meter, the first angular transducer, and the second angular transducer; 
 calculating a second distance extending from the measured three-dimensional coordinates of the first reference retroreflector to the measured three-dimensional coordinates of the second reference retroreflector; 
 calculating a third distance by subtracting the first distance from the second distance; 
 calculating a fourth distance by multiplying the scale factor by the first distance; 
 calculating a discrepancy value by subtracting the third distance from the fourth distance; and 
 taking an action when the discrepancy value exceeds the maximum allowable discrepancy, the action being either issuing an alarm or measuring the three-dimensional coordinates at least some retroreflector targets from among the collection of retroreflector targets and, from this data, re-establishing a frame of reference for an object under test.